Cationic liposomes are of interest as a non-viral vehicle for the delivery of biologically active substances such as drugs, hormones, enzymes, nucleic acids and antigens, including viruses, to cells both in vitro and in vivo. Indeed, cationic liposomes have been demonstrated to deliver genes in vivo (Nabel, E. G., et al. (1990) Science, 249: 1285-1288), (Brigham, K. L., et al. (1989) Am. J. Respir. Cell Mol. Biol., 195-200, Stribling, R., et al. (1992) Proc. Natl. Acad. Sci. U.S.A., 89: 11277-11281), (Plautz, G. E., et al. (1993) Proc. Natl. Acad. Sci. U.S.A., 90: 4645-4649, Stewart, M. J., et al. (1992) Hum. Gene Ther., 3: 267-275). However, the inhibition by serum components of the transfer of nucleic acids by cationic liposomes limits the application of liposomes as a vector for nucleic acids in vivo to regional administrations which avoid exposure to serum.
In addition, stability is a major problem limiting the use of liposomes, both in terms of shelf life and after administration in vivo. Thus, it is desirable to explore the use of other types of lipid dispersions as delivery systems utility for biologically active substances.
U.S. Pat. No. 4,610,888 refers to the use as a drug-delivery system of water-in-oil emulsions in which the volume of aqueous phase ranges from about 0.70%. to about 10.25% of the volume of the lipid components used. However, such water-in-oil emulsions are unsuitable for delivering substances in blood or in other aqueous body tissues.